Support context propagation in http clients

[ilyakaznacheev]

Is your feature request related to a problem? Please describe.
In the case of long requests, it is impossible to terminate them via context cancellation.

Describe the solution you'd like
Add methods with context propagation, e.g.

• ReadContext(ctx context.Context, path string) (*Secret, error)
• ListContext(ctx context.Context, path string) (*Secret, error)
• WriteContext(ctx context.Context, path string, data map[string]interface{}) (*Secret, error)
• etc.

Describe alternatives you've considered
No alternatives in the context of Golang best practices.

Explain any additional use-cases
It can take a long time for a certificate to be generated in the Vault. It may be necessary to abort the request and terminate the job correctly.

Additional context
No change of existing methods/signatures is required. New methods will be added. The only difference is using the context passed to the method as a parameter instead of context.Background().

[ilyakaznacheev]

We need this functionality, so I'm ready to implement if a quick release is possible.

[ilyakaznacheev]

All methods are implemented, with 100% backward compatibility. Waiting for review.

[ilyakaznacheev]

Sorry for the mess, still working on my rebase skills